DS9746 datasheet(8/10 Pages) RICHTEK

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RT9746

DS9746-00 March 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

Power Up

The RT9746 has a threshold of 2.7V power on reset (POR)

with a built-in hysteresis of 100mV. Before the input voltage

reaches the POR threshold, the RT9746 is off. When the

input voltage is over the POR threshold, the VOUT of

RT9746 will delay for 32ms which includes soft-start time

of 8ms. The 32ms delay allows the transient at the input

during a hot insertion of the power supply to settle down

before the IC starts to operate. During the soft-start

transition, the RT9746 slowly turns on the internal

MOSFET to reduce the inrush current.

Over Temperature Protection (OTP)

The RT9746 monitors its internal temperature to prevent

thermal failures. The chip turns off the MOSFET when

the junction temperature reaches 160

°C. The IC will

resume after the junction temperature is cooled down

°C.

Input Over-Voltage Protection

The RT9746 provides input over-voltage protection via

internal or external resistor to set OVP level. If OVPFB is

connected to GND, the RT9746 uses the internal OVP

level setting. If an external resistor-divider is connected to

OVPFB and VOVPFB exceeds the VOVP_SEL voltage, the

RT9746 will adopt external OVP level setting. The OVP

level VOVP is set as below.







OVP

REF_EXT

= V

x 1+

When the input volatge exceeds the OVP level, the RT9746

will turn off internal MOSFET around 0.18us to prevent

the high input volatge from damaging the end system.

When the input volatge returns to normal operation volatge

range with hysteresis (internal 150mV, external 100mV),

the RT9746 will turn on the MOSFET to re-enable output.

Over-Current Protection (OCP)

The RT9746 monitors the output current to prevent the

output short or the charging of the battery with an excessive

current. The RT9746 has a built-in 180

μs delay time to

prevent any transient noise triggering the OCP. If the OCP

situation keeps for 180

μs, the internal MOSFET will be

turned off.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

θJA, is layout dependent. For

WL-CSP-12B 1.92x1.27 (BSC) package, the thermal

resistance,

θJA, is 59.6°C/W on a standard JEDEC 51-7

four-layer thermal test board. The maximum power

dissipation at TA= 25

°C can be calculated by the following

formula :

PD(MAX) = (125

°C − 25°C) / (59.6°C/W) = 1.67W for

WL-CSP-12B 1.92x1.27 (BSC) package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curve in Figure 1 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

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